1. Field
The disclosed concept pertains generally to handle operators and, more particularly, to handle operators which extend beyond a flash protection boundary of, for example, a motor control center.
2. Background
A flash protection boundary (“FPB”) is implemented and regulated to protect those that work around live electrical equipment from severe injury resulting from an arc flash. An arc flash is a type of electrical explosion that can result from a low impedance electrical connection to ground or a voltage phase in an electrical system. For example, when insulation or isolation between electrified conductors is breached or can no longer withstand the applied voltage, an arc flash can occur. An arc flash can cause substantial damage, fire or injury. An enormous amount of concentrated energy can explode outward from the electrical equipment, spreading hot gases, melting metal, causing death or severe burns and creating intense pressure that can damage hearing or brain function and light that can damage eyesight. The fast-moving pressure wave also can send loose material, such as pieces of equipment, metal tools and other objects, flying, injuring anyone standing nearby.
An FPB is calculated to determine the distance surrounding the potential arc point inside which qualified workers must be protected when working. In accordance with the National Fire Protection Association (“NFPA”) 70E standard, FPB is defined as the distance from exposed live parts within which a person could receive a second-degree burn if an electrical arc flash were to occur. This standard also defines incident energy as the amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. Incident energy is expressed in calories per cubic centimeter squared (cal/cm2). As workers get closer to the energized equipment, the energy increases. The FPB is different for different types of equipment and depends, in part, on the voltages involved. Typically, the higher the voltages, the larger the danger zone. At voltage levels above 600 volts, the FPB is the distance at which the incident energy is 1.2 cal/cm2, equating to a second-degree burn. For situations where the fault clearing time is 0.1 second (or faster), the FPB is the distance at which the incident energy level equals 1.5 cal/cm2. This is defined by NFPA 70E 130.3(a), second paragraph, and is the burn level at which the skin will just heal without scaring.
In accordance with NFPA regulations, employers are required to perform a hazard analysis to determine FPBs, to provide appropriate protection for employees and to mark with a warning label electrical equipment having a potential for arc flash. A method of determining this boundary is to calculate the magnitude of the arc (a function of the available short circuit current), estimate how long the arc will last (a function of the interrupting time of the fuse or circuit breaker) and then calculate how far away an individual must be to avoid receiving an incident energy of 1.2 cal/cm2.
The FPB distance can be calculated according to EQ. 1 as follows (in accordance with formulae D.3(d) and D.3(e) Modified of NFPA 70E-2004).FPB=53×MVAbf×T  (EQ. 1)wherein:
FPB represents the flash protection boundary in feet;
MVAbf equals 1.732×V×Isc×0.707/106;
MVAbf represents the bolted fault energy of the arc (MVA);
T represents arcing time (in seconds);
Isc represents bolted short circuit current (in amperes); and
V represents line-to-line voltage (in volts).
All persons crossing the FPB must wear appropriate personnel protective equipment (PPE), such as, but not limited to, protective clothing, for their protection.
In addition to the FPB, there is also a shock protection boundary (“SPB”) which includes a limited approach (“LA”) distance, a restricted approach (“RA”) distance and a prohibited approach (“PA”) distance. The LA distance is the distance an unqualified worker must stay away from energized equipment. The RA distance is the distance that a qualified worker must stay away from energized equipment without voltage rated PPE. The PA distance is the distance considered to be the same as actually touching energized equipment.
An FPB is required around electrical equipment, such as switchboards, panelboards, industrial control panels, motor control centers, and similar equipment, when an individual works on or in the proximity of exposed energized (energized and not enclosed, shielded, covered, or otherwise protected from contact) components. This includes conducting activities, such as examination, adjustment, servicing, maintenance or troubleshooting.
In the case of motor control centers, a motor control center (“MCC”) generally has an assembly of one or more enclosed sections having a common power bus. An MCC can include several motor starters. An MCC is typically used for low-voltage, three-phase, alternating current motors from about 230 volts to about 600 volts. An FPB is calculated for the MCC, and therefore, a worker must typically enter the FPB to perform operations and maintenance on the MCC unit.
The implementation and regulation of FPBs and the provision of PPE afford protection for workers that are required to work in hazardous areas. However, there is room for improvement in removing workers from a hazardous zone. For example, the use of long-handled tools can allow the worker to perform certain operations from outside of the FPB.